It has been important to improve the communication speed in mobile telephone service in recent years because of an expansion of the demand for data communications in addition to voice communications. For example, in a GSM (Global System for Mobile communications) system widespread mainly in the European and Asian regions, hitherto, voice communications have been conducted with GMSK modulation for shifting the phase of a carrier wave in response to transmission data; an EDGE (Enhanced Data rates for GSM Evolution) system for also conducting data communications in 3π/8 rotating 8-PSK modulation (which will be hereinafter abbreviated as 8-PSK modulation) enhancing bit information per symbol to three times relative to GMSK modulation by shifting the phase and the amplitude of a carrier wave in response to transmission data is proposed.
Requirement for linearity for a power amplifier of a wireless communication apparatus transmission section is rigid in a linear modulation system involving amplitude fluctuation like the 8-PSK modulation. Generally, the power efficiency in a linear operation area of a power amplifier is low as compared with the power efficiency in a saturation operation area. Therefore, if a conventional quadrature modulation system is applied to the linear modulation system, it is difficult to increase the power efficiency.
Then, a system of accomplishing higher efficiency of a power amplifier in the linear modulation system, called EER (Envelope Elimination & Restoration) for separating a transmission signal into a constant amplitude phase signal and an amplitude signal, executing phase modulation in a phase modulator based on the constant amplitude phase signal, inputting a constant amplitude phase modulation signal at a level at which the power amplifier becomes a saturation operation point, and driving the control voltage of the power amplifier at high speed for combining amplitude modulation is known (for example, refer to p. 427 and FIG. 7.1 of non-patent document 1). Particularly, a modulation system of separating a transmission signal in a baseband and executing modulation using provided constant amplitude phase information and amplitude information is called Polar Modulation system (polar modulating system) (for example, refer to p. 428 and FIG. 7.2 of non-patent document 1). In the description that follows, it is called polar modulation system to clarify a description of a modulation system different from the conventional quadrature modulation system.
In the polar modulation system, it is difficult at the present technological level to ensure the linearity of the output voltage amplitude relative to the input control voltage of a power amplifier for the required dynamic range for representing amplitude information in output of the power amplifier and therefore it becomes necessary to apply a distortion compensation processing technology.
FIG. 27 is a block diagram to show a polar modulation transmitter in a related art incorporating a predistortion (hereinafter, abbreviated as PD) distortion compensation processing technology described in FIG. 10 of patent document 1.
As shown in FIG. 27, a polar modulation transmitter 20 includes a power amplifier 1, a polar modulation section 2, a distortion compensation processing circuit 3, an amplitude modulation section 10, a phase modulation section 11, and an amplitude and phase measurement section 12. The distortion compensation processing circuit 3 includes delay adjustment sections 4 and 5, memory 6, an address generation section 7, an amplitude information correction section 8, and a phase information correction section 9.
Next, the operation of the polar modulation transmitter 20 in the related art shown in FIG. 27 will be discussed.
To implement a wireless communication apparatus transmission section using the polar modulation transmitter 20, the polar modulation section 2 separates a baseband orthogonal coordinate signal (IQ signal) input from a signal generation section not shown of a wireless communication apparatus into an amplitude signal r(t) and a phase signal θ(t) of a constant amplitude. Here, r(t) is normalized in a predetermined value.
The distortion compensation processing circuit 3 performs predetermined distortion compensation processing for the amplitude information r(t) and the phase information and outputs the amplitude information after subjected to the compensation θ(t) to the amplitude modulation section 10 and also outputs the phase information after subjected to the compensation to the phase modulation section 11. The configuration and the operation of the distortion compensation processing circuit 3 are described later.
The phase modulation section 11 executes phase modulation based on the phase information output from the distortion compensation processing circuit 3.
The power amplifier 1 combines amplitude modulation with a phase modulation signal output from the phase modulation section 11 based on an output signal from the amplitude modulation section 10 as a control signal.
When a predetermined input signal is given to the polar modulation section 2 and a control voltage for decreasing the output signal amplitude of the power amplifier 1 at a predetermined interval from the maximum value is supplied, the amplitude and phase measurement section 12 measures the output signal amplitude characteristic and the passage phase characteristic of the power amplifier 1 for each control voltage value and outputs acquired data to the memory 6.
Next, the configuration and the operation of the distortion compensation processing circuit 3 will be discussed in detail:
To compensate for the differential delay between the paths of the amplitude modulation signal and the phase modulation signal, the delay adjustment sections 4 and 5 give a predetermined delay to the amplitude information and the phase information output from the polar modulation section 2 and output the amplitude information after subjected to the delay adjustment to the address generation section 7 and the amplitude information correction section 8 and output the phase information after subjected to the delay adjustment to the phase information correction section 9.
The memory 6 stores the inverse characteristics of the output signal amplitude characteristic relative to the input control signal, of the power amplifier 1 at a predetermined input high-frequency signal amplitude, output from the amplitude and phase measurement section 12 (AM-AM: Amplitude Modulation to Amplitude Modulation conversion, which will be hereinafter referred to as AM-AM characteristic) and the passage phase characteristic (AM-PM: Amplitude Modulation to Phase Modulation conversion, which will be hereinafter referred to as AM-PM characteristic), and outputs an amplitude correction signal and a phase correction signal of the inverse characteristics of the power amplifier 1 in response to an address signal output from the address generation section 7. The characteristics indicate the characteristics at the time of supplying a control voltage in a steady state.
To distinguish the AM-AM characteristic and the AM-PM characteristic of the power amplifier 1 at the time of supplying a control voltage in a steady state, the inverse characteristics thereof, and the characteristic of the power amplifier at the amplitude modulation operation time from each other, hereinafter the characteristic of the power amplifier 1 acquired using a measurement section such as the amplitude and phase measurement section 12 will be referred to as forward characteristic (AM-AM forward characteristic, AM-PM forward characteristic), compensation data stored in distortion compensation processing memory such as the memory 6 will be referred to as inverse characteristic (AM-AM inverse characteristic, AM-PM inverse characteristic), and the characteristic of the power amplifier 1 at the amplitude modulation operation time will be referred to as dynamic characteristic (AM-AM dynamic characteristic, AM-PM dynamic characteristic).
The address generation section 7 converts the amplitude information output from the delay adjustment section 4 into a discrete value in a predetermined range and having a predetermined step width found from the compensation data stored in the memory 6 and the compensation accuracy and then generates an address signal to reference the compensation data stored in the memory 6.
The amplitude information correction section 8 makes a correction to the amplitude information output from the delay adjustment section 4 based on the amplitude correction signal output from the memory 6.
The phase information correction section 9 makes a correction to the phase information output from the delay adjustment section 5 based on the phase correction signal output from the memory 6.
Thus, the amplitude modulation signal and the phase modulation signal previously distorted considering the inverse characteristic of the output characteristic of the power amplifier relative to the input control signal receive the effects of the actual amplitude distortion and phase distortion occurring in the power amplifier and become any desired output amplitude and phase and the linearity relative to the input control voltage can be improved.
The description of the operation of the polar modulation transmitter in the related art described in patent document 1 is concluded. Hereinafter, the polar modulation transmitter 20 described in patent document 1 will be referred to as the polar modulation transmitter in the related art.
Subsequently, an art required for implementing a wireless communication apparatus transmission section using the polar modulation transmitter in the related art will be discussed.
Considering the operation environment of a wireless communication apparatus, adaptive distortion compensation processing for correcting characteristic change of a power amplifier at the environment variation time of the ambient temperature, etc., is required. The polar modulation transmitter in the related art does not assume distortion compensation processing of updating compensation data during the transmission operation and thus cannot deal with adaptive processing.
Then, there is an adaptive predistortion (hereinafter, abbreviated as APD) distortion compensation processing technology of making a comparison between a baseband transmission signal (input signal) and a signal provided by executing frequency conversion of an output signal of a power amplifier until the baseband (output signal) at a predetermined time interval and correcting an error between the input signal and the output signal according to an LMS (Least Mean Square) algorithm, etc., (for example, refer to FIG. 2 of patent document 2).
In the polar modulation system, it is necessary to ensure synchronization with an accuracy of one several tenth of symbol interval for the differential delay between the amplitude signal and the constant amplitude phase signal. In the APD distortion compensation processing technology, synchronization between the input signal and the output signal also needs to be ensured with the same level of accuracy.    Patent document 1: International Patent Publication No. 2004-501527    Patent document 2: International Patent Publication No. 2002-514028    Non-patent document 1: Kenington, Peter B, Tekiigh-LinearityRF Amplifier Design baiAArtech House Publishers